1. Field of the Invention
This invention relates to an electric dynamometer, and more particularly to an apparatus for detecting torque which can detect accurate torque from counter-voltage in a DC machine.
2. Description of the Prior Art
In conventional DC electric dynamometers for detecting torque, it is customary that a stator of a DC motor or DC generator (hereinafter referred to simply as "DC machine") is constructed so as to be capable of swingly moving around a rotating shaft, and the torque which functions between the stator and a rotor is measured by means of a weighing instrument connected to the stator or a strain detecting gauge attached to a load shaft.
On one hand, a torque T in a DC machine is represented by the following equation (1): EQU T=A.multidot.I.multidot..phi.-T.sub.M ( 1)
where I is armature current, .phi. is field magnetic flux, T.sub.M is mechanical loss torque, and A is constant.
Therefore, the torque T in a DC machine can be calculated, when the armature current I, field magnetic flux .phi., and mechanical loss torque T.sub.M are determined.
It is generally difficult to directly detect the above-mentioned magnetic flux .phi., so that the magnetic flux .phi. has ordinarily been obtained on the basis of the following equation (2): EQU .phi.=B.multidot.(E/N) (2)
where E is counter-voltage, N is speed of rotation of the rotary shaft, and B is constant.
As a consequence, the magnetic flux .phi. can be computed, when the counter-voltage E and speed of rotation N are determined. On one hand, an accurate measurement of the speed of rotation N is comparatively easy, so that an accurate measurement of the counter-voltage E is eventually required to precisely introduce the magnetic flux .phi. from equation (2).
In conventional apparatuses, however, such counter-voltage E could not have precisely been measured so that accurate magnetic flux .phi. has not been determined, and accurate torque could not have been obtained.
That is, heretofore, terminal voltage V of an armature 1 has been measured as illustrated in FIG. 1a, and the terminal voltage V thus measured has been considered to be the counter-voltage E, i.e., E.apprxeq.V in order to simply obtain the counter-voltage E. However, accurate measurement of the counter voltage E cannot absolutely be effected in the manner as stated above. This is because an equivalent circuit for armature winding has a DC resistance component R.sub.a as illustrated in FIG. 1b. More specifically, the counter-voltage E is represented by the following equation (3): EQU E=V-I.multidot.R.sub.a ( 3)
where R.sub.a is DC resistance component of the armature winding, V is terminal voltage of the armature winding, and I is current flowing through the armature winding.
Consequently, as apparent from equation (3), an error corresponding to I.multidot.R.sub.a exists in such system in which the terminal voltage V of the armature 1 is considered to be counter-voltage, so that accurate measurement becomes difficult.
In view of the above, a device as shown in FIGS. 2a and 2b has been proposed as a system by which relatively accurate counter-voltage can be obtained.
That is, FIG. 2a shows a circuit for introducing such counter-voltage, and FIG. 2b shows an equivalent circuit for the circuit of FIG. 2a in which a resistor 2 having resistance value R.sub.2 is connected to an armature 1. Further, resistors 3 and 4 having resistance values R.sub.3 and R.sub.4, respectively, are inserted in the circuit of FIG. 2a in parallel to the armature 1 and the resistor 2. A terminal 5 is inserted in between the armature 1 and resistor 2, while a terminal 6 is inserted in between the resistors 3 and 4. Reference numerals 7 and 8 designate terminals connected to an external power source (not shown), respectively. The principle of the system as stated above will be described hereinbelow in conjunction with FIGS. 2a and 2b.
If it is assumed that voltage V is applied to the terminals 7 and 8, the voltage between both ends of the resistor 4 is VR.sub.4, and the voltage between both ends of the resistor 2 is VR.sub.2, respectively, these voltages VR.sub.4 and VR.sub.2 can be computed in terms of the following equations (4) and (5), respectively. ##EQU1##
Then, the voltage between the terminals 5 and 6 is given by the following equation (6): ##EQU2## where if the respective resistance values are selected so as to become R.sub.3 /R.sub.4 =R.sub.a /R.sub.2, the following equation is obtained: ##EQU3##
That is, when the ratio R.sub.a /R.sub.2 of the voltage between the terminals 5 and 6 is invariable, a value proportional to the counter-voltage E is obtained. As a result, such value is not influenced by the armature current or armature voltage, so that it becomes possible to detect the accurate counter-voltage E.
In practice, however, a value of the DC resistance component R.sub.a of an armature winding fluctuates in response to temperature change followed by the operation, and it results in an error, so that precise detection cannot be attained in respect of counter-voltage. More specifically, the resistance component R.sub.a varies in accordance with temperature change due to the atmosphere or operation of the device. In other words, if it is assumed that the resistance of the armature winding at a standard temperature is R.sub.ao, resistance temperature coefficient is .alpha., and a difference between the standard temperature and a temperature of the armature winding is t, the resistance component R.sub.a of armature winding is given by the following equation (8): EQU R.sub.a =R.sub.ao (1+.alpha.t) (8).
The system for measuring the counter-voltage E according to the device as illustrated in FIGS. 2a and 2b is accompanied with such difficult condition that the ratio R.sub.a /R.sub.2 is invariable, so that the counter voltage E cannot accurately be measured at all times, and accurate detection of torque becomes also difficult.